The standard DF time trial helmet points down in back due to your recline... I think Greg Gross tried one at the TTTT one year, not sure how well it worked, or what he had to do to it. Obviously, you're looking to have a helmet that minimizes the "open clamshell" profile to the wind.
I like my Lazer 02 - I can angle it down somewhat, and it doesn't have the adjustment knob down low in the back, hitting the headrest. However, I don't know of any "recumbent specific" helmets out there. Looking at photos of distance racers like Maria or others, I don't think they try to do much special to minimize helmet angle.
Here's Lynn on her Musashi, with the Air Attack helmet Doug mentioned above:
Here's me at HHH, on the Xstream, alongside Doug - note that Doug sits somewhat more upright in the Velo:
By contrast, here's Kent Polk from a couple years ago, laid out at about 10 degrees... that reclined, I don't think there's much you can do about helmet angle!
Here's Maria Parker, in a marketing photo, for a bit of bike cheesecake